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VHDL, Visual-Basic and VDM

V is for VHDL, Visual-Basic and VDM.

VHDL and Verilog are the leading hardware description languages. My only real hands on experience with digital hardware is soldering together my first computer (buying it in kit+component form saved me a month’s salary), but given the amount of dataflow programming involved I appreciate that programming hardware is likely to be very hard (but not as hard as programming Quantum computers).

The problem with general purpose cpus is that they never seem to have just the right instructions needed to generate really fast compact code for a particular application. The ideal solution is for the compiler to automatically figure out what instructions are the best fit for a given application and then to include the appropriate processor specification in the generated executable (program initialization starts with configuring the host cpu with the appropriate architecture+instructions to execute the generated code). Hardware is available to make this a reality, but the necessary software support is still being worked on (there is also the tiresome economic issue of mass produced cpus almost always being cheaper). At the moment application specific processors tend to be built into application specific computers.

Anyway, real programmers don’t write programs that make LEDs flash on a Raspberry Pi, real programmers use single-board computers that support personalization of the instruction set. If this future has arrived for you, please let me know how I can be included in the distribution.

Visual-Basic shares half of its name and use of the keyword Dim with BASIC. It was clever marketing to give a new language a name that contained the name of a language with a reputation for being easy to learn and use. I bet the developers involved hated this association.

VDM was for many years THE formal method; complete definitions of industrial strength languages (e.g., PL/1, Modula-2 and CHILL) were written using it. These days it seems to be supporting a small community by helping people model industrial systems, however it has fallen out of fashion in the programming language world.

Back in the day there was lots of talk of tools using the available VDM language specifications to check source code (my interest was in checking the consistency of the specifications). In practice tooling did not get much further than typesetting the printed VDM.

Back when I thought that formal methods had a future, I was more of a fan of denotational semantics than VDM. The CHILL VDM specification has at least one reported fault (I cannot tell you where it is because I cannot find my copy); mine was the first reported and I never heard if anybody else ever reported a fault.

Things to read

Computer Architecture: A Quantitative Approach (second edition) by John L. Hennessy and David A. Patterson. Later editions become more narrowly focused on current hardware.

The Denotational Description of Programming Languages by Michael J. C. Gordon is a good short introduction, while Denotational Semantics by David A. Schmidt is longer.

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